Soft Starter Caused

[Ahmet Erecek]

Could anyone one help me? Using a soft starter, can cause a winding failures due to lack of insulation strenght? If yes, what is the reason? For example: because of harmonics?

 

Ahmet ERECEK

[marke]

Hello Ahmet

 

Welcome to the forum.

 

I am not aware of any isues with soft starters that would put any extra stress on the insulation than other forms of reduced voltage starter, in fact I suspect that the soft starter would put less stress on the windings than the open transition star delta starter.

 

Best regards,

[GGOSS]

Hello Ahmet,

 

I would agree with Marke, however have been faced with the situation (several times now) where a customer have been advised by a motor manufacturer that the most likely cause of failure is the soft starter. It is not uncommon for 'some' motor manufacturers to 'shift the blame' when a motor failure occurs.

 

Regards,

GGOSS

[jraef]

I agree.

My belief is that the majority of "motor sellers" have almost no clue as to what a soft starter is or does. But somewhere along the line they heard stories about VFDs causing winding damage and for them, soft starters and VFDs are "all in the family"; the family of strange boxes filled with electronics gizmos they don't understand. I can't even count the number of times I have been told this at a job site by the "motor expert". What I do is challenge them to explain to me how the soft starter COULD damage the motor windings. They almost always come back with "harmonics" because they think they know that harmonic distortion is a mysterious problem that most people don't understand, even though when a soft starter is finished ramping, there are NO harmonics on a continual basis. What is especially humorous is that in most cases, the soft starters are supplied with bypass contactors, so they are not even in the circuit for more than 30 seconds at a time.

 

Ahmet,

VFDs can in fact have detrimental effects on motor windings under the right circumstances. It isn't a direct cause of "harmonics", but the harmonics and the motor damage have common roots: the high speed switching On and Off of transistors in order to simulate a variable frequency-variable voltage sine wave. The motor winding damage is done by a phenomenon called "standing wave generation" which can actually develop into voltage pulses going to the motor that are up to 3 times the potential of the line voltage! You can Google "VFD standing wave" for more information if interested.

 

For purposes of this discussion however, suffice it to say that soft starters do NOT use high speed transistors, they use relatively slow SCRs which are turned on, but turn themselves off naturally at the zero-cross point of each sine wave. As a result, the SCR switching creates very little electrical noise and/or harmonics. They also do not create standing waves, so they do not do anything that can damage the motor windings. In addition, as mentioned earlier they are only in the circuit during ramping, 5 to 30 seconds typically. Compared to the hours of running time (during which the SCRs are no more active than the contacts of a contactor), the ramp time is insignificant as far as harmonics (or anything else) goes.

 

If someone is telling you this, they do not know what they are talking about.

[jOmega]

All......

 

Should we assume therefore, that THERMAL STRESS would not be a consideration...????

 

 

jΩ

[jraef]

All......

 

Should we assume therefore, that THERMAL STRESS would not be a consideration...????

 

 

jΩ

 

Thermal stress is always a problem. But a properly commissioned soft starter should not put any additional thermal stress on a motor than any other form of starting, (with the exception of a VFD). So from that stand point it is not the "soft starter" causing the damage, it is an error in setting it up or using it properly. Another common mistake people make is in thinking that if my motor is having trouble starting across-the-line, I can fix the problem by adding a soft starter. This of course is exactly the opposite of what intuition should have told them, but nonetheless I have seen it done time and time again. Those people then think it was the soft starter's fault as well. In reality they never should have used a soft starter when what they really needed was a bigger motor!

 

Besides, the OP specifically mentioned "insulation strength" which I would interpret as a voltage issue, not a thermal one.

[jOmega]

re: jraef Posted Sep 24 2006, 05:08 PM

 

JRaef......

 

Well stated. Having seen more than one motor failed as a result of improper set-up

of a Soft Starter, your comments cannot be overstressed.

 

Permit me, if you will, one point of clarification; I think you will find that motor manufacturers

will define the property of Insulation Strength as the ability to stand up to thermal stress

as well as voltage (dv/dt) stress and mechanical stress.

 

While a brand new motor may have Class F insulation when it is shipped from the factory, over time, that insulation will degrade.... At some point in the future, after experiencing hundreds of DOL starts, the user may decide to add a soft-starter to it and not realize that the insulation has degraded .... I counsel customers to take that into account when considering a soft-starter solution to an older motor. Reducing the starting current but extending the starting time .... can exceed the motor's existing, degraded thermal capability

 

Kind regards,

jΩ

[jraef]

 

... Permit me, if you will, one point of clarification;...

 

OK, permitted!

 

You are right of course, but generally when people ask me questions such as that, they are thinking voltage issues, mostly because of what they have experienced with VFDs.

 

You make an interesting point about retrofitting soft starters onto motors already stressed from DOL stating, and I agree. I sometimes have referred to soft starters as the "weakest link detector" (especially after the TV game show of that name), because the slower acceleration often unmasks previously unobserved probelms such as broken rotor bars and bearing problems as well. I'll have to add "degraded insulation" to that list.

[GGOSS]

I would accept everything stated above however I my experience most failures are experienced with new or relatively new motors. Another interesting observation is that once the motor has been replaced or repaired, long term reliable operation is realised. Does this not further support the argument that a manufacturing defect was the most likely cause of motor failure?

 

Regards,

GGOSS

 

PS: Ahmet, see also the discussion thread "maximum start current with soft starters"

[jOmega]

I would accept everything stated above however I my experience most failures are experienced with new or relatively new motors. Another interesting observation is that once the motor has been replaced or repaired, long term reliable operation is realised. Does this not further support the argument that a manufacturing defect was the most likely cause of motor failure?

 

Regards,

GGOSS

 

PS: Ahmet, see also the discussion thread "maximum start current with soft starters"

 

 

G'day GG---,

 

Do you have any historical data that you could provide in support of your statement.

I have NO doubts as to the veracity of your statement; rather, an interest in learning more

about the failures you have observed.

 

For instance, could advise any of the below detail ?

 

√- power rating and quantiy in each rating

 

√- mfgr. and model number of failed motors

 

√- type of failure; i.e.,

 

√√√- Stator winding - in-slot arced over to stator laminations

 

√√√- Stator winding - in-slot - turn-to turn short (insulation punch-thru)

 

√√√- End-turn - winding-to winding (turn-to-turn)

 

√√√- End-turn winding - phase-to-phase

 

√ - Mechanical Failure

 

√ - Application

 

√ - How many hours from first application of power (commissioning) to failure occurence ?

 

I know that after-the-fact, it is difficult to reconstruct such data ... but the data would be very informative in attempting to understand why you have seen more failures in new motors as opposed to legacy motors.

 

Thanks, and kind regards,

jΩ

 

p.s. With regard to your question, 'statistically speaking' the sample size and lack of such detail as requested above make it statistically impossible to support the conclusion that majority of failures are attributal to mfgr. defects.

[GGOSS]

Hello jOmega,

 

Please accept my sincere apology for this late reply, have been suffering from a dose of the flu over the last month. Will attempt to answer your questions as they are raised.

 

8 in total, 150 - 265kW 2 pole motors, 3 different sites accross Australia.

 

Mix of Toshiba & Teco.

 

Turn to Turn shorts in the slots.

 

Screw compressors, unloaded starts.

 

Most within 3 months of first power up, remainder within 1 year of first power up.

 

Although all failures appeared identical, only on one occassion the motor manufacturer advise manufacturing defect ie crimped paper insulation.

 

Although I would accept your argument about statistics, the information provided was based on my own experiences gained over my 24 years in electronic motor starting. I have never once identified or been made aware of a motor failure that can be directly attributed to it being controlled via a soft starter. I do agree (and have already stated) that poor commissioning can present a potential for failure. Such failures however could also occur through the use of an incorrectly selected or commissioned electro-mechanical RVS.

 

regards,

GGOSS